1. Field of the Invention
The present invention relates to a porous metal sheet composed of a layered structure, a battery electrode plate composed of the porous metal sheet, and a battery having the electrode plate. More particularly, the present invention relates to a porous metal sheet that is wound spirally to accommodate it in a cylindrical battery can of a secondary battery such as a nickel cadmium battery, a nickel hydrogen battery, and the like.
2. Description of the Related Art
As a positive plate and a negative plate for use in the nickel cadmium battery, the nickel hydrogen battery, and the like, porous metal sheets are hitherto used. The porous metal sheet is formed by plating a foamed three-dimensional reticulate porous material such as a polyurethane sponge; a nonwoven cloth; and a mesh net.
Of the above porous metal materials, a foamed metal sheet, which is formed by plating the polyurethane sponge and then burning the polyurethane, has a higher porosity than a punching metal and the like. Thus, the foamed metal sheet can be charged with a larger amount of an active substance than other porous metal sheets. Further, pores are three-dimensionally present in the foamed metal sheet. Thus, the foamed metal sheet holds the active substance therein in a high extent. Therefore, the porous metal sheet consisting of the foamed metal sheet is preferably used as an electrode substrate.
In the conventional electrode substrate composed of only the foamed metal sheet, needless to say, the number of cells on one side (surface) of the electrode substrate is almost equal to the number of cells on the other side thereof. In the case of the electrode substrate composed of the foamed metal sheets disposed on its both sides and a mesh metal sheet or a nonwoven metal sheet sandwiched between the foamed metal sheets, the number of cells of the foamed metal sheet disposed on its one side is almost equal to that of cells of the foamed metal sheet disposed on its other side. This is because both-side foamed metal sheets are formed by plating the urethane sponge made of the same material.
The electrode substrate composed of the foamed metal sheet is charged with the active substance to form an electrode plate. The rigidity of the electrode substrate is increased by charging it with the active substance. When the electrode plate is accommodated in a cylindrical can, which has a small inner diameter, by spirally winding it, it is wound at a very small curvature to accommodate it in the cylindrical can in a required volume or more than the required volume. Thus, normally, the electrode plate is so wound that the inner diameter of an innermost spiral circle of the electrode plate is as small as about 3 mm. Consequently, as shown by the photograph of FIG. 7A and the illustration of FIG. 7B, the rigid electrode plate is susceptible to crack. Because the electrode plate has a certain thickness, the elongation percentage of the outer periphery thereof is higher than that of the inner periphery thereof. Thus, a crack C and a breakage B are liable to occur from the outer periphery of the electrode plate. With the recent demand for the development of a miniaturized battery and for increase in its capacity, this type of crack and breakage occur more frequently than before.
With reduction of the strength of the electrode plate relative to a tensile force, the electrode plate is susceptible to small cracks but is not susceptible to a large crack or breakage when it is wound. As a method of reducing the strength of the electrode plate, it is conceivable to increase the number of cells of the foamed sheet and to increase the number of frameworks surrounding the cells (pores). Supposing that the same amount is deposited on a unit area of a foamed sheet, the amount deposited on a surface area-increased framework becomes small. Consequently, the foamed metal sheet has a narrow framework and hence a low strength. However, when a resulting foamed metal sheet is charged with the active substance while it is being successively fed, the foamed metal sheet cannot be charged with the active substance efficiently at a high feeding speed because it has a low strength relative to a tensile force. Further, when the foamed material is plated while it is being successively fed, it cannot be plated efficiently at a high feeding speed as in the case of the time when the foamed metal sheet is charged with the active substance. Further, when the number of cells is increased, it is difficult to plate the inner side of the foamed material made of the urethane sponge, because the diameters of the cells are small. In this case, the foamed material is plated non-uniformly.
On the other hand, as a method of increasing the strength of the electrode plate, it is conceivable to decrease the number of cells of the foamed material to reduce the surface area of the framework surrounding the cells. Supposing that the same amount is deposited on a unit area of a foamed material, the amount deposit on a surface area-decreased framework becomes large. Consequently, the foamed metal sheet has a thick framework and hence a high strength. Therefore, it is possible to charge the resulting foamed metal sheet with the active substance efficiently at a high feeding speed. However, because the foamed metal sheet has a high strength relative to the tensile force, it is difficult to wind the foamed metal sheet when the foamed metal sheet is wound at a small curvature. Further, the foamed metal sheet cracks occur deep into the inner peripheral side thereof and is thus susceptible to breakage. The broken portion of the foamed metal sheet may break through a separator.
The present invention has been made in view of the above-described situation. Thus, it is an object of the present invention to prevent a electrode from cracking deeply and broken when it is spirally wound, allow a winding work to be accomplished smoothly, and prevent a feeding speed from being reduced when it is charged with an active substance.
In order to achieve the object, according to the present invention, there is provided a porous metal sheet, for use in a battery electrode substrate, composed of a two-layer structure consisting of a first foamed metal layer and a second foamed metal layer or a structure having three or more layers consisting of the first foamed metal layer, the second foamed metal layer, one or more metal layer sandwiched between said first foamed metal layer and said second foamed metal layer and selected from a group consisting of a mesh metal layer, a nonwoven metal layer or/and a metal sheet layer having uniform pattern pores formed therethrough. In the porous metal sheet having the construction, the number of cells of the first foamed metal layer and that of the second foamed metal layer are different from each other.
The number of cells of the second foamed metal layer is more than that of the first foamed metal layer by specifying the number of cells of the first foamed metal layer to 30 PPI or more and less than 60 PPI and specifying the number of cells of the second foamed metal layer to 50 PPI or more and less than 80 PPI.
The number of cells of the first foamed metal layer is set to 30 PPI or more and less than 60 PPI for the following reason: if the number of cells of the first foamed metal layer is less than 30 PPI, i.e., because the number of cells thereof is too small, the first foamed metal layer has a small active substance-holding force. Further, because the diameters of cells are large, the utilization percentage of the active substance deteriorates because the active substance is not electrically conductive. If the number of cells of the first foamed metal layer is 60 PPI or more, it is difficult to allow the mechanical property of the first foamed metal layer to be different from that of the second foamed metal layer. The reason the number of cells of the second foamed metal layer is set to 50 PPI or more is because if the number of cells of the second foamed metal layer is less than 50 PPI, it is difficult to generate small cracks on the second foamed metal layer. On the other hand, if the number of cells of the second foamed metal layer is 80 PPI or more, the second foamed metal layer has a too low tensile strength.
It is preferable to set the number of cells of the first foamed metal layer to 45 PPI or more and less than 55 PPI and the number of cells of the second foamed metal layer is more than 56 PPI or more and less than 65 PPI.
PPI is a unit indicating the number of cells (pore) per inch. For example, 30 PPI means that the number of cells per inch is about 30, and 80 PPI means that the number of cells per inch is about 80. When the number of cells per inch is as small as 30, the number of frameworks surrounding pores is small, and thus the surface area of the framework per inch is small. When the number of cells per inch is as large as 80, the number of frameworks surrounding pores is large, and thus the surface area of the framework per inch is large. Supposing that the same amount is deposited on a unit area of a foamed material, the foamed material, which has a decreased number of cells to reduce the surface area of its framework, has a larger deposit amount and a larger deposit thickness on its framework. Further, because the foamed material has large-diameter cells, the foamed material made of the urethane sponge can be plated uniformly and inwardly. Therefore, the framework surrounding the cells is thick and thus has a high strength relative to a tensile force. On the other hand, the foamed material, which has a large number of cells and a surface area-large framework, has a less deposit amount on its framework and thus a deposit thickness is small. Further, because it has small-diameter cells, it is difficult to plate the foamed material inward. Therefore, the framework surrounding the cells is thin and thus has a low strength relative to the tensile force.
It is preferable to use polyurethane sponge having a thickness of 0.5 mm-3.0 mm and a pore diameter of 200 xcexcm-800 xcexcm as a base material of the both-side foamed metal layers. It is preferable to use metals Cu, Ni, Ni alloy, Zn, Sn, Pd, Pb, Co, and Fe as the metal to be used to plate them.
The above-described porous metal sheet is produced as follows: initially, foamed materials of urethane sponge or the like having different number of cells are layered on each other and layered surfaces thereof are bonded to each other with an adhesive agent or welded to each other; or they are layered on each other with a mesh, a nonwoven cloth, and/or a metal sheet having pores of uniform pattern sandwiched between the foamed materials and layered surfaces thereof are bonded to one another with the adhesive agent or welded to one another. Then, the layered structure is plated. Then, it is sintered and the resin is removed by burning it. In this manner, the porous metal sheet having the foamed metal layers having different number of cells formed at both sides thereof is produced.
The present invention provides a battery electrode plate composed of an active substance-applied porous metal sheet having foamed metal layers, formed at both sides thereof, having different number of cells, different strengths and different elongation percentages relative to a tensile strength.
The present invention provides a battery accommodating an electrode plate in a cylindrical can. The electrode plate is spirally wound with the second foamed metal layer located on the outer side thereof and with the first foamed metal layer located on the inner side thereof.
As described above, in the present invention, the electrode plate is wound, with the second foamed metal layer located on the outer side thereof. This is because the second foamed metal layer has more cells and narrower framework and thus has a lower degree of strength and a lower elongation percentage relative to a tensile force than the first foamed metal layer. Further, as described previously, the outer side of a circular plate has a higher elongation percentage than the inner side thereof. Therefore, the second foamed metal layer is very susceptible to a lot of small cracks. Owing to the generation of the many small cracks, the electrode plate can be wound smoothly. Further, because a lot of small cracks are generated on the outer side of the electrode plate, it is difficult for a crack generated on its outer side to reach its inner side. That is, the electrode plate can be prevented from being destroyed. Furthermore, in feeding the porous metal sheet to charge it with an active substance to form the electrode plate, it is possible to feed the porous metal material at a high speed by pulling it at a high degree of force because the first foamed metal layer having a higher strength relative to a tensile force is disposed at the inner side of the porous metal sheet. Thus the productivity of the electrode plate can be improved.